Also in the CSS Pane rules view, CSS variable names will now auto-complete (bug 1422635). If you enter var( into a property value and then type a dash (-), any variables you have declared in your CSS will then appear in an autocomplete list.

The view_source.tab preference has been removed so you can no longer toggle View Source mode between appearing in a new tab or new window. Page sources will always appear in new tabs from now on (bug 1418403).

HTML

Pressing the Enter key in designMode and contenteditable now inserts <div> elements when the caret is in an inline element or text node which is a child of a block level editing host — instead of inserting <br> elements like it used to. If you want to use the old behavior on your app, you can do it with document.execCommand(). See Differences in markup generation for more details (also see bug 1430551).

Canvas and WebGL

CSSOM

No changes.

HTTP

No changes.

Security

The X-Content-Type-Options header, when set to no-sniff, now follows the specification for JavaScript MIME types. In particular, text/json and application/json are no longer valid values (bug 1431095).

Plugins

No changes.

Other

Fetches that include credentials can now share connections with fetches that don't include credentials. For example, if the same origin requests some web fonts as well as some credentialed user data from the same CDN, both could share a connection, potentially leading to a quicker turnaround (bug 1363284).